DE 22 33 700 C2 discloses coating hard metal substrate bodies consisting of a mixture of at least one metal serving as a bonding agent and at least one very hard metal carbide of high with a layer of aluminum oxide or zirconium oxide. Particularly the substrate body can be made of tungsten carbide, titanium carbide, tantalum carbide or niobium carbide, or, a compound carbide consisting of tantalum and niobium, whereby the metals cobalt, iron or nickel serve as bonding agent. In the literature hard metals based titanium carbide or titanium carbonitride are frequently mentioned as cermets, which are also to be understood as substrate materials, just like the pure combinations of a hard metal with ceramic, i.e. nonmetallic, components. According to the DE 22 33 700 C2 the mentioned layers of .alpha.-aluminum oxide are applied by means of CVD at substrate temperatures of 1000.degree. C.
A corresponding layer may be applied to the hard metal bodies according to the DE 22 53 745 A1, which consist of a sintered hard metal substrate body, intermediate layers of titanium carbide and an outer layer of aluminum oxide, whereby the first titanium carbide intermediate layer has to be applied at 1000.degree. C. and the second aluminum oxide layer at 1110.degree. C. by a CVD process. As mentioned especially in DE 28 25 009, column 2, lines 28 and following, hard, polycrystalline and compact .alpha.-aluminum oxide layer are normally produced only at deposition temperatures of 950.degree. C. At lower deposition temperatures, according to the state of the art, loose and powdery depositions are obtained, which consist of the .gamma. modification and/or .alpha. modification of the aluminum oxide. At deposition temperatures of approximately 1000.degree. C. and above, the aluminum oxide phase is normally the .chi.-modification considered suitable for the coating of tools. In order to prevent the danger of obtaining multiphase aluminum oxide coatings, which presumably occur at temperatures below 1000.degree. C. and which show a considerable mechanical weakness causing premature tool failure, it is proposed that the coating of aluminum oxide consist entirely or at least in proportion of 85% of the .chi.-modification and that an optional remainder consisting of the .alpha.-modification form areas or spots not exceeding a size of 10 .mu.m on the surface. For the deposition the CVD process at temperatures of approximately 1000.degree. C. is suggested. However, this type of coating tends to convert into the .alpha.-modification under the action of high temperatures, so that cracks appear in the layer, leading to premature failure especially under the action of hot gas corrosion.
In order to avoid the problems occurring at high deposition temperatures, the DE 32 34 943 discloses the deposition of an amorphous aluminum oxide layer. However, intensive testing with amorphous aluminum oxide layers, deposited by means of PVD, have shown that purely amorphous aluminum oxide layers have a glass-like breaking behavior, and therefore can not offer any significant improvement in wear resistance. In the case of interrupted cuts, these layers tend to split.
In the DE 24 28 530 A1 a process for the protection of a metallic part against corrosion and wear is proposed, which in the pure or alloyed state contains at least one element of the Group I B of the periodic system of elements and wherein on the surface of this part a layer of amorphous and transparent aluminum oxide is deposited by chemical deposition from the vapor phase. However the amorphous layers applied at temperatures between 300.degree. and 800.degree. C. are far less stable when subjected to thermal influences than for instance the modification of the aluminum oxide (.alpha.-Al.sub.2 O.sub.3) known as corundum.
Basically it is also known to use aluminum oxide layers as protective layers against hot gas corrosion, for instance in internal combustion engines. In this case special requirements exist not only with regard to the mechanical stability of the layer, but also to the density of the layer. According to the state of the art this can be achieved only by comparatively thick (approximately 500 .mu.m) ceramic layers produced through thermal spraying.